1. Field of the Invention
The present invention relates to a flow-rate measuring method for measuring the flow rate of a fluid and to a flow-rate measuring device.
This application is based on Japanese Patent Application No. 2008-033445, the content of which is incorporated herein by reference.
2. Description of Related Art
In a known method for measuring the flow rate of a fluid, a portion of the fluid is intentionally given a temperature that is different from that of the rest of the fluid (this portion will be referred to as a “thermal marker” hereinafter), and a thermal-marker detecting unit monitors the movement of the thermal marker so as to measure the flow rate of the fluid (Japanese Unexamined Patent Applications, Publication Nos. 2002-148089 and 2004-271523).
In the methods disclosed in Japanese Unexamined Patent Applications, Publication Nos. 2002-148089 and 2004-271523, the thermal marker can be generated from the outside of a tube through which the fluid flows by using a heat source, such as a heater or a laser. This means that the heat source is not brought into direct contact with the fluid. Likewise, the thermal-marker detecting unit uses, for example, a laser beam as detection light for detecting a change in the absorbance of light of a specific wavelength in the fluid so that the thermal-marker detecting unit can measure the flow rate without being in direct contact with the fluid. This is advantageous in that, when measuring the flow rate, the fluid can be prevented from being contaminated as a result of coming into contact with undesirable materials.
On the other hand, although the sensitivity to a change in the absorbance of light of a specific wavelength caused by a temperature change in the fluid is satisfactory in the methods disclosed in Japanese Unexamined Patent Applications, Publication Nos. 2002-148089 and 2004-271523, the characteristics of the heat source are not satisfactory for generating a thermal marker. The reason is that, in a method where the heat source, such as an external heater, is installed in direct contact with the exterior of the tube through which the fluid flows, there is a time lag in heat transmission since the fluid itself is heated after the heat from the external heater is transmitted to the tube, thus resulting in a heated region which is undesirably wide. This spreads the pattern of the thermal marker wider than the heat source and thus leads to an increase in a flow-rate measurement error.
For solving this problem, a heating method performed by irradiating a microscopic region of the fluid with a laser beam is proposed. In such a heating method using a laser beam, a specific wavelength of the laser beam is selected, and the laser beam is transmitted through the tube through which the fluid flows, whereby the fluid can be directly heated while the heated portion of the fluid is limited to a laser-beam irradiation region thereof. In consequence, the thermal marker is prevented from over-spreading, thus preventing the flow-rate measurement accuracy from being reduced.
However, since there are no low-cost high-output lasers suitable for this purpose at present, only low-output lasers can be used under the present circumstances. Due to the low-output characteristics of these lasers, a temperature change in the fluid is limited to a low value, resulting in a limited measurable flow-rate range. Therefore, the measurable flow-rate range is limited to 10 ml/min or lower, and the difference between the upper limit and the lower limit of the measurable flow-rate range is limited to a factor of ten.